Joint degeneration is a common problem that can occur in a variety of joints throughout the human body. The condition typically is more prevalent as the skeletal system ages and is often treated with medications and/or physical therapy. These conservative treatments sometimes meet only limited success. If unsuccessful, the patient typically will continue to experience ongoing pain and limited mobility.
Often the treatment progression leads to a total joint replacement. These replacements have been performed for years in joints such as the hip and the knee. The replacement devices usually comprise some form of a metallic structural component or endplate with an intermediate polyethylene core. It is not unusual for replacements such as these to give 15-20 years of service before requiring some degree of revision.
In the spine, the surgical treatment of choice has been fusion for the treatment of intervertebral disc degeneration. The spinal intervertebral disc is arguably the most important joint in the spine and is situated between the vertebral bodies. The spinal disc is comprised of a tough outer ring called the annulus, and a jelly-like filling called the nucleus. The belief has been that removing the diseased spinal disc(s) and fusing between affected levels will not make a significant difference in the overall mobility of the spine. However, spinal fusion has proved to cause an increase in degeneration at other vertebral levels that must compensate for the loss of motion at the fused level commonly causing the patient to relapse into more pain and limited mobility.
Recently, there has been a focus on the use of “motion preservation” implants over implants that promote spinal fusion. These motion preserving implants, in the form of joint replacements in the spine, hope to alleviate many of the problems associated with fusion devices in the spine. Intervertebral disc replacement devices are seen today typically comprising a pair of biocompatible metal plates with a polymer or elastomeric core, or a metal plate articulating on a metal plate.
Metal on metal implants have a history of failure in long term use, however, precision machining has spawned a reemergence of implants using these materials since it is believed that this change in manufacturing greatly improves the wear. Regardless, the metal implants are radiopaque and continue to frustrate surgeons due to the difficulty in imaging the affected area. Other implants, such as those using a polymer or elastomeric core between metallic plates suffer from the same radiopaque frustrations due to the metal components in addition to the added complexities of design due to the necessity of utilizing a multitude of materials for a single implant.
The prior art discloses a laundry list of biocompatible materials including metals, ceramics, and polymers, that can be used for the manufacture of these implants, yet historically many of these materials have failed when interfaced together and tested in an articulating joint. There is in particular an extensive history of failure when polymers articulate against polymers in weight bearing artificial joints. Due to this failure history, polymer combinations have naturally been excluded as an acceptable self-articulating material combination for use in weight bearing joint replacements.
PEEK (poly-ether-ether-ketone), for example, has been suggested as an appropriate material of manufacture for use in implant devices due in large part to its strength, radiolucent nature, and biocompatibility. This is particularly true in structural implants having no articulating component. PEEK on PEEK has been suggested for use in low wear non-weight bearing joints such as in finger joints. However, the prior art has been careful not to suggest self-articulating PEEK on PEEK as a suitable material combination in weight bearing joint replacement devices due to the failure history of biocompatible polymers articulating against themselves.